The present invention relates to a voice and data multiplexing system and in particular to a voice and data multiplexing system including a device for coding and compressing voices in which the voice and data signals are simultaneously transmitted in an efficient manner when voice and data frames which are multiplexed with each other are transmitted over the same communication line, and a recording medium on which a program used for multiplexing a voice and data is recorded and from which it is readable with a computer.
Multiplexing schemes have been developed for simultaneously transmitting voice and data signals over the same transmitting line. One of the examples may include a DSVD (Digital Simultaneous Voice and Data protocol) modem.
The DSVD modem is adapted to implement transmission of multiplexed voice and data signals by establishing a plurality of logical channels on an analog public line for transmitting voice code frame and data signal frames over different logical channels.
Both voice and data frames in the DSVD are transmitted as information frames which are defined by ITU-T recommendation V.42. In order to allocate voice and data frames to different logical channels on transmission, the DLCI (Data Link Connection Identifier) field value is set to 0 and 32 for data and voice frames, respectively, for transmission.
In an exemplary condition on transmission of multiplexed voice and data frames, the DLCI field value is specified and a voice frame and a data frame are transmitted over one and the same physical channel.
In order to efficiently use the restricted band width on a transmission line for transmission of voice signals, voice codes are periodically compressed with a compressing algorithm and the compressed voice codes are transmitted in the form of voice frames. Since the voice frames are used for transmission of conversation between both parties in this case, a multiplexer transmits voice frames preferentially to data frames so that conversation will not become intermittent.
On the receiving side, the compressed voice codes which are contained in received voice frames are decompressed for decoding voice signals.
When the input voice signal represents silence, the period of time in which the transmission line is occupied is shortened by transmitting a frame having a size which is smaller than that when a voice is present by using voice codes representing silence.
Since the data frames are transmitted in an interval between voice frames, the transmission rate of data frames changes with the changes in transmission rate of voice frames. The transmission is identical with that of the general V-series modem except that the transmission rate is dynamically changed.
In an exemplary configuration of the prior art DSVD modem, a transmission control unit, a multiplexing unit, a separating unit, voice signal compressing unit, a voice signal decompressing unit, a data terminal control unit, a voice signal coding unit, a voice signal decoding unit, a data terminal for transmitting and receiving data, a voice input device (microphone) such as telephone hand set and the like and a voice output device (speaker) such as telephone hand set and the like are provided with.
In the DSVD modem having such a configuration, the voice signal to be transmitted, which is input from the voice input device such as a microphone of the handset of a telephone set is coded in the voice signal coding unit and the coded signal is periodically compressed in the voice signal compressing unit.
A frame of the compressed voice codes to be transmitted is multiplexed with a frame of data to be transmitted which has been sent via the data terminal control unit and the multiplexed signal is transmitted to a communication line via the transmission control unit.
At this time, in the multiplexing unit a busy signal which informs that the voice signal is preferentially transmitted so that the transmission line is in a busy state is provided to the data terminal control unit. The data terminal control unit will inform the data terminal that the line is in a busy state in response to the given busy signal for performing flow control between the unit and the data terminal.
When the input voice is silent, a silence frame which is shorter in length than the frame when a voice is present is transmitted as a voice frame to shorten the period of time in which the transmission line is occupied.
The frames which are received from the public line are passed through the transmission control unit to the separating unit, in which they are separated into received voice and data signals. The received voice signal is decompressed in the voice decompressing unit and is decoded in the voice decoding unit and is fed to the voice output device such as a headphone of the handset of the telephone set.
On the other hand, the received data signal which has been separated by the separating unit is fed to the data terminal via the data terminal control unit.
Although the transmission rate of the data frame can be enhanced by transmitting a silence frame having a smaller size during a silent period of time in the abovementioned scheme, it is wasteful that the silence frames are repeatingly transmitted during the silent period of time since the silent time is continued for an extended period of time in the general conversation in comparison with the period of compression and coding of voice signal.